CAD-CAM techniques have already been adopted in the field of dentistry for some time, and are replacing the traditional manual production of dentures. The currently usual machining production methods for producing ceramic dental restoration bodies have, however, some disadvantages which cannot be improved according to the current prior art with acceptable outlay within economic constraints. In this context generative production methods, known by the term “rapid prototyping”, may be envisaged, in particular stereolithographic methods in which a newly applied material layer is respectively polymerized in the desired shape by position-selective exposure, so that the desired body is produced successively by layered shaping in its three-dimensional shape, which is defined by the sequence of applied layers.
Filled photopolymerizable materials, in particular ceramic-filled materials, are important as materials to be processed for dental restorations. In relation to the processing of ceramic-filled photopolymers, reference may be made for example to the prior art according to WO 98/06560 A1. In the method described therein, a ceramic slurry is exposed through a dynamic mask (light modulator) and thereby cured, so that a three-dimensional shaped body can be constructed successively layer by layer. In the described method, the ceramic slurry is exposed from above on a production platform. With such exposure from above, a new thin material layer must be applied with the aid of a blade after each exposure (typically with a layer thickness which lies between 10 and 100 μm). For highly viscous photopolymerizable materials, such as ceramic-filled resins are, thin layers can however be applied reproducibly only with difficulty in this way.
A method of the type mentioned in the introduction is described in WO 2010/045950 A1 and corresponding US Published Application No. US2011310370, which is hereby incorporated by reference. The method is used for the layered construction of a shaped body by using lithography-based generative manufacture, for example rapid prototyping. A defined layer of photopolymerizable material, which is contained in a tank having a horizontal bottom formed so as to transmit light at least in subregions, is formed in the following way. A production platform which is vertically moveable in a controlled manner is supported by a lifting mechanism and is arranged on the tank so that it can be raised and lowered by the lifting mechanism under the control of a control unit. By lowering the production platform into the photopolymerizable material in the tank, material is displaced from the gap between the lower side of the production platform and the tank bottom. By accurate setting of the vertical position of the production platform, a layer of photopolymerizable material with an accurately defined layer thickness can thus be produced between the lower side of the production platform and the tank bottom. The layer of photopolymerizable material defined in this way is then exposed in the desired geometry by position-selective exposure from below through the light-transmitting tank bottom, so as to cure the layer on the production platform. The production platform with the first layer cured thereon is subsequently raised and the exposed region is replenished with photopolymerizable material, since the material cannot readily flow back from the surrounding regions of the tank into the exposed region. The production platform is then re-lowered, so as again to define a layer of photopolymerizable material with a predetermined layer thickness between the lower side of the cured layer and the tank bottom. These steps are repeated so as to construct layer by layer the shaped body consisting of successive layers, each with a predetermined geometry.
After a layer has been cured, the production platform with the part of the shaped body already formed thereon is raised. In the region exposed to form the last layer, a free space or “hole” then remains over the tank bottom, since the material previously contained there in the defined layer of photopolymerizable material has been cured by the last exposure and raised vertically with the production platform. In the case of highly viscous filled photopolymerizable material, in particular oxide ceramic-filled or glass ceramic-filled polymers, the problem arises that the resulting “hole” in the exposed region must be refilled with photopolymerizable material since, owing to its high viscosity, the highly viscous material cannot readily flow back from the surrounding regions as would be the case with unfilled photopolymerizable materials. To this end, in WO 2010/045950 and corresponding US Published Application No. US2011310370, which is hereby incorporated by reference, a blade is provided and is moved relative to the tank with a predetermined distance from the lower edge of the blade to the tank bottom, so as to move photopolymerizable material from regions outside the region in the tank exposed last into the free space remaining after raising the last cured layer. Here, the blade functions as a displacement element in order to transport photopolymerizable material into the free space left behind; it is not, however, used in order to define the layer thickness since the layer thickness of the layer to be formed next is set by lowering the production platform with the shaped body part adhering thereon into the photopolymerizable material to a predetermined distance from the tank bottom. The use of a blade for moving highly viscous photopolymerizable material in order to refill the previously exposed region has not proven effective.